The present invention is directed to electrical amplifiers. The present invention has especial relevance to Class D amplifiers. Electrical amplifiers inherently generate noise during their operation. Class D amplifiers generally have higher noise than Class A-B amplifiers because of the added complexity of circuitry embodying a Class D amplifier contributing additional noise sources along signal paths.
To reduce noise in an amplifier such as a Class D amplifier, preamplifier noise may be shifted up in frequency by chopping the analog audio signal being amplified. To create a pulse width modulated (PWM) output signal, the audio signal is compared with a saw tooth ramp. If frequency of the chopping signal and the ramp signals are substantially the same frequency or if one of the chopping signal and the ramp signal is substantially a multiple or a harmonic of the other signal, the preamplifier noise can be mixed back down to the audio frequency band, the band of the audio signal being amplified. As a result of such mixing down to the audio band, reduction of noise by employment of a chopping signal is not accomplished to the amount that could otherwise be achieved.
There is a need for an apparatus and method for effecting chopping in an amplifier device that avoids mixing down noise to the frequency band being amplified by the amplifier device.